What's your benchmark? If you have access to the hardware virtually everything can be jailbroken. With the iPhone, however, you can't do it non-destructively (i.e. you can't get personal information). At least that's my understanding, if I'm wrong please correct me.
The original user-friendly jailbreaks worked by pressing a button on a website. That means that any website could use the same technique to run native code with root privileges on your iPhone. I'm not sure how much physical access is required for current jailbreaks.
IIRC it's been years since that's been possible, recent attempts seem to all fake a software update/reset via host computer connection. I'm not saying the iPhone is anywhere impossible to break into (I'm sure, like all OSs, there are vulnerabilities that allow access exposed to the user that have not been found yet), but because the apps are sandboxed, hackers need to break into software and then out of the sandbox (probably into kernely memory, again, but possibly just into privileged system access to the file system). The iPhone is certainly far more secure than most desktops because of this, and I suspect that because of Apple's control over it, the chance of your iPhone getting accessed without your permission is miniscule.
However, I am no mobile developer, so take this with a grain of salt.
